An atom has a central nucleus and
electrons in resonant shells around the nucleus. Heavier atoms such as gold have
more shells than lighter atoms such as carbon. Flooding an atom with
brilliant light of all colors the electrons begin to jump to higher energy
shells. When the excitation, which could be light or electron bombardment,
is removed the electrons begin to jump back to lower energy shells. These jumps
between shells release 'quanta' of energy. Rather than light being released as
in a rainbow the light is released at precise frequencies or colors. The
precise colors or spectral lines change to other spectral lines if the
excitation energy is increased or decreased. The spectral lines are of a
very precise frequency and each chemical element has it's own set of spectral lines. The
spectral lines can be used to measure chemical composition, energy
level and Doppler shift. See reference
below.

Returning to our simple spectrograph as shown
above we see the ring nebula focused on the slit by the telescope primary
mirror. Widening the slit will allow more light to enter but will blur the
spectral lines so that lines that are close may be blurred together. Most spectrographs
have a camera focused on the slit so the astronomer can tell which
portion of the sky is entering the slit.

The Original Diffraction Grating

The diffraction
grating effect was noticed when looking through fine cloth at a point light
source. In 1820 Fraunhofer made the first
diffraction grating by winding thin wire over two fine screws of equal pitch
kept an inch or two apart by bars of metal. Using these wire gratings Fraunhofer
was able to measure the wave length of yellow sodium light.

Modern gratings are
made by ruling lines on glass using a diamond point and more recently good
plastic gratings have been molded from glass originals.

More about the
Spectrograph

For more precise spectral measurements
better spectral resolution is required. Light from distant galaxies is faint to
image and putting the light through the spectrograph adds to the problem. Large
telescope primaries and large diffraction gratings are required. The telescope
guidance system must be precise and stable to be useable by the astronomer.

The Echelle High
Resolution Spectrometer

Visit the echelle simulation drawings.

Note that with an Echelle spectrometer
rainbow colors overlap. More than one wave length of light will add at a
particular angle from the grating. The different colors at the same angle are
called 'orders'. First order, second order, etc. The frequency of the different
orders are 'harmonics' of one another. Harmonic frequencies are frequencies that
are related mathematically as common fractions are related by a 'common
denominator'. To separate the orders from one another a second low dispersion
grating or prism is required. The second disperser is called the 'cross
disperser'.

The echelle spectrometer shown uses a
lens for the collimator and a prism for cross dispersion. Prisms are not too
popular in modern spectroscopy because a prism spreads the blue light much more
than the red light. The prism requires a 'non linear' scale when reading
spectral lines. Sometimes the orders can be isolated with color filters. Modern
multilayer dielectric filters can be designed to isolate particular bands of
color while rejecting other color bands.

Below is the layout for one type of
echelle spectrometer taken from the book 'Astronomical Optics' by Daniel J.
Schroeder.

Build and Calibrate your own CCD Spectrograph

'Sky and Telescope' July, 2000,
page 125 has an article about an amateur built Spectrograph.